The invention relates to a transducer comprising a diaphragm on which the mechanical quantity to be measured acts. The deformation of the diaphragm is being converted to an electric signal by means of strain gauges. The membrane or diaphragm is mounted to a reinforced clamping rim. The strain gauges are attached to the outside of the diaphragm or to a flexure spring connected to the diaphragm.
Such transducers comprise, for instance, a diaphragm, to which strain gauges are directly secured which respond to an appropriately strong action of the mechanical quantity to transform the respective change of the deflection of the diaphragm into an electric resistance change. Pressure transducers, for instance, convert a pressure in a gaseous or liquid medium into a resistance change in the strain gauges. For this purpose such pressure transducers are provided with a diaphragm or with a spring plate which is customarily equipped directly with thin-film strain gauge strips to respond to rated pressures of 10 bar or more. Four strain gauge strips are generally connected to form a full bridge circuit, with two opposite strain gauges of the bridge being stressed in one sense by the deformation of the diaphragm, for instance, in compression, and being located closer to the clamped rim of the diaphragm than the other two oppositely stressed, for instance, tension strain gauges, which are arranged in the center of the diaphragm, or at any rate, farther away from the clamping rim of the diaphragm than the first-mentioned strain gauges. The arrangement and mode of operation of such transducers is independent of the mechanical quantity. Stated differently, it does not matter whether the quantity to be ascertained is a force, or a pressure or an acceleration or a moment or torque. However, where pressures are to be ascertained the transducer would be provided with a diaphragm having a reinforced center.
If the mechanical quantities acting on the transducer are small, the device carrying the strain gauges, in the present case, a diaphragm or a flexure spring, must of necessity also be made thinner as the acting quantity decreases. Here, also two of the four strain gauge strips connected to form a full bridge circuit, are arranged closer to the rim than the other two.
If a supply voltage is applied, heat or Joule heat is generated in the strain gauges. With the usual supply voltage of 10 V and the usual strain gauge resistance of 350 ohms, the generated heat may be sufficiently large so that temperature differences may occur in the diaphragms due to the relatively poor heat conduction of the materials used for such diaphragms, for instance, alloy steels. The diaphragms are heated by the strain gauges and the heat generated in this manner is conducted through the diaphragm to the clamping rim. The strain gauge strips, which are arranged at different distances from the clamping rim, assume different temperatures because, although the same amount of heat is generated in each strain gauge strip, the heat transfer from the strain gauge to the clamping rim is different. Such different temperature of the strain gauge strips leads to different resistance changes of the strain gauge strips due to the thermal coefficient of the electric resistance. Depending on the manner in which the strain gauge strips are connected to form a full bridge circuit, these resistance changes are cummulative and lead to a bridge unbalance.
A temperature equilibrium is reached in the diaphragm only relatively slowly. The transducers then exhibit a zero drift or zero variation upon switching-on which, for instance, with a 10 V supply voltage, may continue for several minutes. This can result in measuring errors of the order of, for instance, 5% of the nominal value. If the thermal coefficient of the electric resistance depends, in addition on the temperature, then the temperature response of the null or zero point will also depend on the supply voltage.
The same difficulties are encountered if strain gauges are attached to a flexure spring coupled to a diaphragm. Here again, two strain gauge strips of a full bridge circuit must be arranged closer to the clamping rim, where the heat conduction is better, while two strain gauge strips are located farther away from the clamping rim, at a point where the heat transfer to the rim is lower.